1. Field of the Invention
The present invention relates to an acoustic apparatus including a resonator.
2. Prior Art
A speaker system as one type of acoustic apparatus is arranged such that a speaker unit (vibrator) is disposed in a cabinet and is driven by an amplifier (AMP). Of reproduction characteristics of the speaker system, low-frequency reproduction characteristics are mainly determined by the volume of the cabinet.
A dynamic direct radiator speaker (dynamic cone speaker) as a typical direct radiator type speaker has a substantially conical diaphragm. The diaphragm is driven by a voice coil in a magnetic gap attached near the top of the cone. When such a speaker is used in the acoustic apparatus, a direct sound is radiated from the front surface of the diaphragm, and acoustic waves are also radiated from its rear surface. The acoustic waves from the front and rear surfaces have opposite phases. Therefore, if a difference in propagation distance of the acoustic waves from the front and rear surfaces to a listener is almost an odd multiple of a half wavelength, sound pressures from these surfaces are in phase with each other, and are superposed.
However, if the difference in propagation distance of the acoustic waves is almost an even multiple of the half wavelength, the sound pressures cancel each other and are attenuated. Thus, taking into consideration the fact that sounds having various wavelengths are radiated from the speaker, it is preferable that the sound from the rear surface does not reach the listener or the sound from the rear surface does not adversely influence the direct radiation sound from the front surface.
For this purpose, the direct radiator type speaker employs a baffle. As a baffle for shielding communication of sounds from the front and rear surface of the diaphragm, a plane baffle, back-opening cabinet type baffle, closed baffle, and the like are known, as shown in FIGS. 29A to 29C. Furthermore, as a baffle having a slightly different purpose than the above baffles, a phase inversion type (bass-reflex type) baffle shown in FIG. 31 is known. These baffles will be described below.
FIG. 29A is a sectional view of a plane baffle. A hole having the same size as a vibrator is formed in a single, wide flat plate 1, and a substantially conical diaphragm 2 is mounted in this hole. A dynamic electroacoustic transducer (speaker) 3 including a voice coil, a magnetic circuit, and the like is attached to the top portion of the cone of the diaphragm 2. According to this plane baffle, since a sound from the rear surface is shielded by the plate 1, if the plate 1 is assumed to have an infinite size, a perfect baffle effect can be obtained. However, a plate having an infinite size is not realistic, and in practice, a plate 1 having a finite size is used. If a minimum frequency of sound pressure reproduction characteristics is set to be about 60 Hz, the plate 1 must be a 2.times.2 (m) square, and cannot be put into a practical use.
FIG. 29B is a sectional view of a back-opening cabinet type baffle As shown in FIG. 29B, a hole is formed in a cabinet 4 open at its rear end and a vibrator constituted by a dynamic speaker 3 having a diaphragm 2 is mounted in this hole. However, according to the back-opening cabinet type baffle, the speaker system must have a large size in order to obtain a necessary baffle effect. An air column in the cabinet 4 constitutes a resonance system, and impairs a transient response.
FIG. 29C is a sectional view of a closed baffle. As shown in FIG. 29C, a hole is formed in the front surface of a closed cabinet 5, and a vibrator constituted by a dynamic speaker 3 having a diaphragm 2 is mounted in this hole. With this structure, if the cabinet 5 can be perfectly prevented from being vibrated, the sound from the rear surface of the diaphragm 2 can be perfectly enclosed, thus obtaining a perfect baffle effect. However, air enclosed in cabinet 5 serves as an air spring, and gives an elasticity to the diaphragm 2. As a result, a resonance frequency as a whole undesirably becomes higher than that of the plane baffle.
This principle will be explained below with reference to FIG. 30. FIG. 30 shows a simplified electric equivalent circuit of the system shown in FIG. 29C. In FIG. 30, reference symbol R.sub.v denotes a DC resistance of a voice coil of the vibrator, and m.sub.o, S.sub.o, and S.sub.c have the following relationships:
m.sub.o : equivalent mass of vibration system PA1 S.sub.o : equivalent stiffness of vibration system PA1 S.sub.c : equivalent stiffness of cabinet
Reference symbol A denotes a force coefficient, which is given by A=Bl.sub.v where B is the magnetic flux density in a magnetic gap and l.sub.v conductor length of the voice coil. A parallel resonance circuit Z.sub.1 by an equivalent motional impedance of the unit vibration system and an equivalent motional impedance A.sup.2 /S.sub.c of the closed cabinet are connected in parallel with each other, and the parallel circuit is connected in parallel with an amplifier (not shown) through the voice coil resistance R.sub.v as a non-motional impedance.
As can be seen from this electric equivalent circuit, a resonance frequency f.sub.oc of a whole system is increased to be higher than a minimum resonance frequency f.sub.o of the vibrator and is given by: EQU f.sub.oc =f.sub.o (1+S.sub.c /S.sub.o).sup.1/2
An equivalent Q value (Q.sub.oc) of the whole system at the resonance frequency f.sub.oc has the following relationship with respect to a Q value (Q.sub.o)of the vibrator itself at the minimum resonance frequency f.sub.o and is increased as: EQU Q.sub.oc =Q.sub.o (1+S.sub.c /S.sub.o).sup.1/2
Therefore, in order to improve low-frequency reproduction characteristics, the equivalent stiffness S.sub.c of the cabinet must be decreased For this purpose, a large cabinet must be employed.
A bass-reflex type speaker system has a slightly different purpose from the above-mentioned baffles. FIGS. 31A and 31B are a perspective view and a sectional view of the bass-reflex type speaker system. As shown in FIGS. 31A and 31B, a hole is formed in a cabinet 6, and a vibrator consisting of a diaphragm 2 and a dynamic speaker 3 is mounted in this hole. An opening port 8 having a sound path 7 is arranged below the vibrator. In a conventional bass-reflex type speaker system, a resonance frequency f.sub.op caused by an air spring in the cabinet 6 and an air mass of the sound path 7 is set to be lower than the minimum resonance frequency f.sub.o of the vibrator (speaker) which is assembled in the bass-reflex type cabinet. At a frequency higher than the resonance frequency f.sub.op caused by the air spring and the air mass, the sound pressure from the rear surface of the diaphragm 2 has an opposite phase in the sound path 7, and hence, the direct radiation sound from the front surface of the diaphragm 2 and the sound from the opening port 8 are consequently in phase with each other, thus increasing the sound pressure. As a result, according to an optimally designed bass-reflex type speaker system, the frequency characteristics of an output sound pressure can be expanded below the low-frequency resonance frequency of the vibrator. As indicated by an alternate long and two short dashed curve in FIG. 32, a uniform reproduction range can be extended wider than those of the infinite plane baffle and the closed baffle.
However, when uniform reproduction is realized by the bass-reflex type speaker system, various limitations are posed on the Q value of resonance of a unit vibration system and the like, and only when these limitations are met, the characteristics shown in FIG. 32 can be obtained. In this manner, in the conventional bass-reflex type speaker system, it is very difficult to obtain an optimal design condition.
On the other hand, an attempt is made to intentionally extremely reduce the resonance frequency f.sub.op of the resonator regardless of the basic design idea of the bass-reflex type speaker system and paying attention to only an acoustic radiation power from the opening port.
However, since the bass reproduction power is closely related to the volume of the cabinet, a larger cabinet must be employed in any event in order to achieve low-frequency reproduction, as is the case with a closed baffle. This situation will be explained in detail below with reference to FIG. 33.
FIG. 33 shows a simplified electric equivalent circuit of the bass-reflex type speaker system shown in FIG. 31. In FIG. 33, reference symbols A, R.sub.v, m.sub.o, S.sub.o, and S.sub.c are the same as those in FIG. 30, and m.sub.p corresponds to an equivalent mass of the sound path (port). A parallel resonance circuit Z.sub.1 by an equivalent motional impedance of the unit vibration system and a series resonance circuit Z.sub.2 by an equivalent motional impedance of a port resonance system are connected in parallel with each other, and this parallel circuit is connected in parallel with a driving amplifier (not shown) through the voice coil resistance R.sub.v as non-motional impedance.
As can be seen from this electric equivalent circuit, the bass-reflex type speaker system includes two resonance systems according to its major characteristic feature. The impedance characteristics of this speaker system present a double-humped curve having a total of three resonance points, i.e., two maximum peaks and one minimum peak therebetween. The resonance point of the minimum peak corresponds to the port resonance system (the above-mentioned closed baffle has only one resonance system, and its impedance characteristics exhibit a single-humped curve including only one resonance point). In the bass-reflex type speaker system, the voice coil resistance R.sub.v of the vibrator (unit) serves as both a damping resistance of the parallel resonance circuit Z.sub.1 of the vibrator side and the series resonance circuit Z.sub.2 of the opening port (duct) side. For this reason, the parallel and series resonance circuits Z.sub.1 and Z.sub.2 mutually interfere with each other.
As an example of mutual interference or mutual dependency, if a vibrator having a strong magnetic circuit is used, a resonance Q value of the vibrator is reduced, while the resonance Q value of the opening port is increased. In contrast to this, if a vibrator having a weak magnetic circuit is used, the opposite situation to the above occurs. In an essential design of the bass-reflex type speaker system, an optimal point capable of obtaining uniform low-frequency reproduction characteristics must be selected under the conflicting mutual dependency condition.
Assume that the volume of the cabinet is reduced. In this case, the minimum resonance frequency f.sub.o of the unit vibration system exhibits the same tendency as that of the closed baffle, and as a result, is increased. The low-frequency reproduction characteristics will finally come to be improved to some extent by the acoustic radiation effect of the opening port. However, if the size of the cabinet is reduced, it cannot be avoided that the low-frequency reproduction power will be decreased as the whole system even in the bass-reflex type speaker system.
In particular, when the resonance frequency f.sub.op of the port resonance system is intentionally decreased from basic setting, as described above, the opening port must be more elongated as the cabinet is smaller in size. Therefore, the Q value becomes very small due to an increase in mechanical resistance of air in the port. An extreme decrease in resonance Q value leads to loss of the acoustic radiation power from the opening port. As a result, the function of the opening port as a resonance duct is lost, and the presence of the opening port becomes meaningless. That is, if the size of the cabinet is reduced, bass reproduction is essentially impossible.
As has been schematically described above, in the conventional acoustic apparatus, various countermeasures are taken in order to allow low-frequency reproduction.
The plane baffle, back-opening baffle, and closed baffle shown in FIGS. 29A to 29C are designed such that radiation sounds from the rear surface of the diaphragm do not reach a listener in front of the speaker system as unnecessary sounds. However, in order to improve the bass reproduction characteristics with these baffles, the apparatus (cabinet) will inevitably be made large in size, and even if it is made so to a certain feasible extent, its low-frequency reproduction characteristics will be insufficient.
In the bass-reflex type speaker system shown in FIGS. 31A and 31B, the phase of the backward sound is inverted by the opening port, so that in particular, a bass range of a direct radiation sound from the front surface of the diaphragm is compensated for. For this reason, the resonance system which is originally very hard to deal with is undesirably formed on the two portions, i.e., the diaphragm and the opening port. In order to obtain a satisfactory bass-reflex effect according to the basic setting, the optimal condition of the system must be very critically set while taking the mutual dependency condition of these two resonance systems. Although various attempts have been made in this respect as disclosed in Japanese Patent Publication No. sho 46-12670 and Japanese Utility Model Publication No. sho 54-35068, these attempts could not eliminate difficulty on design.
In order to improve the low-frequency reproduction characteristics, in any way, the cabinet undesirably becomes bulky.
In some bass-reflex type speaker systems, the resonance frequency f.sub.op of the port resonance system is intentionally decreased from its basic setting. However, if the size of the cabinet is to be reduced, the port resonance system will not significantly contribute to acoustic radiation, thus incurring a fatal drawback.
Therefore, when a bass reproduction power of a certain level or higher is to be obtained according to any of the prior arts, the resulting cabinet will inevitably become large in size. As a result, it is difficult to employ an acoustic apparatus having a cabinet of a proper volume and excellent low-frequency reproduction characteristics in a variety of applications such as in halls, rooms, vehicles, and the like.